1. Field of the Invention
This invention relates to lactide/caprolactone copolymers, a method of making the same, composites of the same with carbon fibers, and prostheses produced from such composites.
2. Description of the Prior Art
The treatment of injured ligaments and tendons remains a serious clinical problem. Inadequately repaired damage to these structures results in pain, loss of function, and in some cases, subsequent degenerative arthritis. When severly damaged by trauma or disease, fibrous tissue repair is often impossible. Many researchers have suggested the use of replacement structures for such damaged tissue. At this time, however, a completely successful prosthesis for use in a chronic implantation has not been developed.
It has recently been demonstrated by Jenkins et al, "Induction of Tendon and Ligament Formation by Carbon Implants", J. Bone and Joint Surg., 59-B:53-57, 1977, and Wolter et al, "Ligament Replacement in the Knee Joint with Carbon Fibers Coated with Pyrolytic Carbon", Trans. 3rd Ann. Mtg., Soc. for Biomat., 126, 1977, that ligaments and tendons can be replaced by filamentous carbon implants. New fibrous tissue grows and is gradually aligned, replacing the carbon scaffold which fractures and degrades mechanically.
Alexander et al, "Carbon-Polymer Composites for Tendon and Ligament Replacement", Trans. 4th Ann. Mtg., Soc. for Biomat., 123, 1978, have indicated the need for physically protecting the delicate carbon fibers to avoid difficulty in implantation, premature fracturing in vivo and migration of carbon fibers from the site of surgery.
Kulkarni et al, "Polylactic Acid for Surgical Implant", Arch. Surg., 93, 839-843, 1966, and Cutright et al, "Tissue Reaction to the Biodegradable Polylactic Acid Suture", Oral Surg., 31: 134-139, 1971, have demonstrated the biocompatibility, biodegradability and ease of manufacture of surgical appliances of polylactic acid polymers.
U.S. Pat. Nos. 4,127,902 and 3,971,670 describe structures for in vivo implantation as substitutes for ligaments and tendons comprising a bio-compatible film, a bio-compatible fabric having a weave with no permanent yield in one direction integral with the film and a bio-compatible porous material which promotes the ingrowth of living tissue. The structure is used as a patch for repairing damaged ligaments and tendons and is designed to promote the growth of new ligament and tendon tissue. The patch, however, is intended for permanent implantation in the host animal.
U.S. Pat. No. 3,276,448 discloses the concept of coating a non-absorbable fiber-containing fabric intended for use as a repair structure for damaged tissue with collagen. The collagen is said to serve as a stimulus for new tissue growth in the area of repair.
It has been proposed in U.S. Pat. No. 3,992,725 to utilize carbon fibers as in vivo implantation material due to its ability to foster new tissue growth by virtue of its bio-compatibility. The patent proposes to combine the carbon fibers with polytetrafluoroethylene bonding material to provide a relatively permanent implant material.
U.S. Pat. No. 3,463,158 discloses the use of composites of polyglycolic acid and non-absorbable fibrous material as implants for tissue repair or replacement. The composition is designed such that new tissue growth surrounds the non-absorbable fibrous material.
U.S. Pat. No. 3,893,196 describes a material for fabricating a prosthesis comprising graphite fibers embedded in a coating with a medically inert plastic.
U.S. Pat. No. 3,272,204 discloses an absorbable collagen prosthetic implant reinforced with strands of non-absorbable material.
U.S. Pat. Nos. 4,045,418 and 4,057,537 describe lactide/caprolactone polymers wherein the lactide is present in major amount. The polymers are taught to be bio-degradable. More particularly, U.S. Pat. No. 4,057,537 discloses copolymers of L-(-)-lactide and epsilon caprolactone wherein the concentration of L-(-)-lactide and epsilon caprilactone which is heated to form copolymers is in the range of about 50 to about 90 wt % based on the total mixture, preferably, about 75 to about 90 wt % (column 3, lines 10-48). Example 1 of this patent discloses the preparation of a 50/50 copolymer of L-(-)-lactide/epsilon caprolactone which is characterized as gummy and having a low tensile strength.
U.S. Pat. No. 3,268,487 discloses a process for the polymerization of lactides.
U.S. Pat. No. 3,531,561 discloses surgical sutures prepared by extruding high molecular weight polylactide polymers. Comonomers may be included in the polymer in minor amounts, e.g., 5-15% by weight.
U.S. Pat. No. 3,636,956 discloses absorbable surgical sutures prepared by the extrusion of polylactide polymers wherein the polymer may contain up to 35 mole % of a glycolide.
U.S. Pat. No. 3,839,297 discloses high molecular weight co-polymers of lactide and glycolactide which may be extruded to make absorbable surgical sutures.
U.S. Pat. No. 4,300,565 discloses sterile surgical articles fabricated from synthetic absorbable copolymers formed by co-polymerizing glycolide monomer with a cyclic ester monomer other than glycolide, lactide being preferred.
As is apparent from the aforesaid references, many absorbable polymers are known and the use of filamentous carbon as an implant material is not unique. Indeed, it has been demonstrated that new fibrous tissue growth is encouraged by the carbon filaments, with the new tissue gradually aligning and replacing the carbon scaffold which fractures and degrades mechanically. However, filamentous carbon is usually produced on a polymer base, often with the addition of polymer sizing agents. These polymers frequently exhibit adverse tissue reactions or are carcinogens; as is polyacrylonitrile, a commonly used base material. It has been suggested to remove the sizing agent with methyl ethyl ketone possibly leaving trace polymer material behind. Complete removal of the sizing and base residue by heating to 4000.degree. F. results in a strong material that is, unfortunately, brittle and sensitive to shear and bending deformations.
In addition, unprotected carbon has been found to break up during implantation and migrate from its implantation area. In some cases, it forms sinus tracks right through the skin.
The mere mechanical reinforcement of the carbon fibers with other materials does not satisfactorily eliminate the migration problem.
Recently, in U.S. Pat. No. 4,329,743 for Bio-Absorbable Composite Tissue Scaffold, there has been disclosed a bio-compatible composition for fabricating a surgical article for the repair or replacement of a part of the body of a human or non-human animal comprising a composite of a bio-absorbable polymer and at least one substrate of a plurality of carbon fibers. Suitable bio-absorable polymers are polyglycolic acid, polylactic acid and collagen. By enveloping the carbon fiber substrate with a bio-absorbable polymer migration of the filamentous carbon after implantation is prevented, without interfering with the new tissue growth promoting characteristics of the carbon fiber substrate. The polymer functions as a mechanical reinforcer for the carbon fibers during tissue growth, and the polymer degrades allowing new tissue growth to replace it, thereby allowing a transference of load from the composite to the new tissue over an extended period of time.
However, the polylactic acid polymers disclosed therein are quite rigid and even when plasticized with, for example, polyethylene glycol, the coated carbon fiber tow is very rigid and the carbon fibers break when bent. Moreover, the plasticizer tends to decrease the adhesion between polymer and fibers.
A need therefore continues to exist for polymers and composites thereof which offer the advantages of polymer bio-absorbability, composite flexibility and good adhesion to the carbon fibers by the polymer without damage to the carbon fiber substrate.